The microbubble with a considerably small diameter to be mixed with the liquid exhibits various functions, the usage of which has been under development. For example, JP-A No. 2007-24012 discloses the application of a microbubble generator to a system for supplying the fuel to the engine of the vehicle. In JP-A No. 2007-24012, the fuel is fed to the microbubble generator outside the tank by the pump inside the tank via the fuel supply passage. The fuel then passes through the ultrasonic generator so as to be supplied to the fuel injection unit.
The generated microbubble with respect to the fuel becomes uniform to be mixed therewith. The temperature of the fuel just before supply to the fuel injection unit may be instantaneously uniformized through irradiation of the ultrasonic wave to the microbubble.
The microbubble generator disclosed in JP-A No. 2007-24012 is markedly large in size, and is required to be disposed separately from the fuel pump, resulting in the layout difficulty. It is difficult to mount such a generator in the vehicle with the limited layout space, for example, with respect to a motorcycle. Additionally, the generally employed microbubble generator is quite expensive.